In a parcel-sorting system, a lamp shines on parcels moving along a conveyor and an LDR receives the reflected light. Parcels for one delivery route carry a reflective patch. When one of these patches passes beneath the lamp, more light reaches the LDR. The LDR and a fixed resistor are connected in series across a supply with a constant potential difference. A switching unit opens a sorting gate when the current rises above a set value.
Explain how the passage of a reflective patch results in the sorting gate opening.
An engineer exchanges the fixed resistor for one of lower resistance. The supply potential difference and the current setting are not changed. Explain what happens to the minimum light intensity needed to open the gate.
A different design would open the gate every few seconds using a timer, with no LDR. Give one advantage of the LDR design for sorting the parcels.
82 exam-style questions on AQA GCSE Physics 2.1 Current, potential difference and resistance, covering 2.1.1 Standard circuit diagram symbols, 2.1.2 Electrical charge and current, 2.1.3 Current, resistance and potential difference, and 2.1.4 Resistors. Each one has a worked solution and a mark scheme showing where the marks go.